1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine having a noble-metal chip joined to an electrode adapted to perform spark discharge.
2. Description of the Related Art
Conventionally, spark plugs have been used for providing ignition in internal combustion engines. Such a spark plug generally includes a metallic shell that holds an insulator in which a center electrode is provided in an inserted condition, and a ground electrode whose one end portion is joined to a front end portion of the metallic shell and whose other end portion is opposed to a front end portion of the center electrode. The spark plug further includes a noble-metal chip for enhancing resistance to arc-induced erosion. The noble-metal chip is provided in either a region of the center electrode or a region of the ground electrode, the regions opposing each other.
Iridium is proposed as a material for such a noble-metal chip (refer to, for example, Patent Documents 1 and 2). Because of increasing demand for enhancing erosion resistance of spark plugs in recent years, iridium, whose melting point is higher than that of conventionally used platinum, is used as a material for noble-metal chips so as to enhance erosion resistance. Generally, such a noble-metal chip is formed into a cylindrical shape, and its one end surface (hereinafter, also referred to as a “proximal end surface”) is joined to the center electrode or the ground electrode, whereas its other end surface (hereinafter, also referred to as a “facing end surface”) is opposed to the center electrode or the ground electrode (in the case where a noble-metal chip is joined to each of the center electrode and the ground electrode, the noble-metal chips are opposed to each other), thereby forming a spark discharge gap therebetween.    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. H09-7733    [Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. H10-22053
3. Problems to be Solved by the Invention
Iridium-containing noble-metal chips are usually formed by the steps of mixing material powders; melting the resultant mixed powder; forming a billet from the molten metal; subjecting the billet to a rolling process and a forming process so as to form the billet into a rod; and cutting the rod into pieces having an appropriate length. The rolling process causes crystal grains of such a noble-metal chip to extend in the axial direction of the noble-metal chip. Accordingly, the crystal grains assume the form of fibers having a larger length along an axial direction of the noble metal chip than along a direction perpendicular to the axial direction, thereby exhibiting superb resistance against oxidation consumption.
However, since the crystal grains of a noble-metal chip have the form of fibers extending in the axial direction, the noble-metal chip is prone to cracking or chipping in the axial direction. Studies conducted by the present inventors have revealed that, when subjected to an external force, a noble-metal chip that contains iridium in an amount of 70% by weight or more does not readily absorb external force through deformation and is prone to cracking or chipping. When such cracking or chipping causes a reduction in the area of the distal (or facing) end surface, which partially forms a spark discharge gap of the noble-metal chip, spark discharge is concentrated on the remaining portion of the distal end surface. As a result, arc-induced erosion of the noble-metal chip is accelerated, and thus the spark discharge gap is prone to increase.